The upgrade counterpart of the MS406, the D520 was a call for quicker planes and unmatched firepower. Thereforth, the D520 in more capable in dog-fighting with its great improvement. The frame of the D520 is also reinforced with steel, making it more resistant to bullets. It is still at threat from pounding AA guns, however.
In response to a specification for a new fighter promulgated by the l'Air Ministry on 15 June 1936, the design of the D.520 started in September 1936, at the private design firm led by Émile Dewoitine. The specifications called for a maximum speed of 500 km/h (310 mph) at 4,000 m (13,000 ft), the ability to climb to 8,000 m (26,000 ft) in less than fifteen minutes, with take-off and landing runs not exceeding 400 m (1,300 ft). The armament was to be two 7.5 mm (.295 in) machine guns and one 20 mm Hispano-Suiza HS.9 cannon, or two HS.9 cannon.
Dewoitine had been disappointed with the performance of his last design, the Dewoitine D.513, which was rejected by the Armée de l'Air in favour of the Morane-Saulnier M.S.406, and decided to respond to the specifications with a design using the latest construction techniques and the most powerful available engine, the new 660 kW (890 hp) Hispano-Suiza 12Y-21 liquid-cooled engine. The first design was rejected by the l'Air Ministry, which, after being impressed by the British Hawker Hurricane and Supermarine Spitfire, then uprated the specifications to include a maximum speed requirement of 500 km/h (310 mph). In response, Dewoitine renamed the further development, the "D.520".